With a bell! They’re cheap and incredibly useful. While you can get along just fine without one, it’s always a good idea to have a clear audible signal readily available.
Yelling “heads up” works sometimes, but it’s not as reliable as flicking your thumb. For example, if you happen to be breathing out when that pedestrian steps in front of you, the time it takes to summon a yell may be a few fractions of a second too long.
A verbal warning can also be misunderstood—remember, not everyone speaks English. The clear “ding!” of a bicycle bell, however, is nigh universally synonymous with “watch out!”
If that’s not enough reasons for you, this post I wrote has one more that you might not have thought of.
So go get yourself a bell already! Emily has ones with dinosaurs on ‘em for only $7. They’re safe and educational!